Servers by jDrones

Hexacopter wants to lean over before liftoff


(MickeM) #21

What to do with those offsets? Why are they there? Is it like the controller thinks it is rotated but the copter is not? Is it because we start at a slightly leaning ground?

Flight controller is in the center of the multicopter, Pixhawk is level, it is also level-calibrated in MP. With artificial horizon, do you mean “the green ground” in the hud in MP? It is level when the multicopter is level. Trims are 0 0 0 0.

ESCs are calibrated using Mission Planners own function. It is useless in informing what is going on and when it is done. The controller beeps, all motors beeps. How dou you know is was successful? The multicopter flies very well though.

If we put the multicopter on a slightly leaning ground, not much, and then arms it, the motors idle normally. Then we give very little throttle and the motors revs upp very high instantly. The motor on the lowest arm revs up more than the others. The more down the arms are, the more their motors revs up. Why doesnt it rev up softly from 0-50 if we do this in for example 2 seconds? What is the point of jumping up to almost liftoff?

Log-file from a short flying from asphalt with a jumpy start (it “vibrates” some in the start), then flies good.
https://1drv.ms/u/s!ArRUW2agVpZnjFLxRtRQEonc26EF


(MickeM) #22

We also use 14" in that flight. RCOUT (PMW OUT) for 1-6 was 75% with 13" and now it is around 68%. 15" next.


(Mike Boland) #23

Taking off from unloved ground takes quite a knack.
That would account for what you are seeing.
At idle it will not try to level itself but the FC sees it is not level and winds up the values.

Consider this:
You arm the copter on un-level ground,
from that moment on it wants to get level,
but the throttle is not high enough for lift so it increases its efforts,
until you have given it enough power to lift,
but it has over compensated due to the time it was not level.
Flip

There is your problem.


(MickeM) #24

I dont get it.

I have been taking of with a lot of multicopters throtugh the years from not so level places, mostly APM/Pixhawk. When revving up the copter starts to level when reaching higer rpms (throttle stick at around 40-50%. This one jumps up immediately. What have changed? Why doesnt it slowly rev up. It wont fly away with only 15% throttle, or 20… or 30. It revs up to almost liftoff-rpms. Is it because of insane Autotune-PIDs? Must be? No flip problems at all. Other than that personality it works splendid and have flown a bunch of missions with good results. Just wonder why it revs up that insanely fast to the high rpms. There is no use for it. =)


(Oldgazer) #25

Solutiion: Arm and take off “smartly.”


(MickeM) #26

Well, the hexacopter works fine.
Take offs are made from a wooden board to not get stuck to the ground or have grass messing with the lens.
Take offs are made certainly and smooth.

But I still cant get the two weird things…

  1. It sometimes start to vibrate “violently” and jumps around the wooden board (legs some cm up). Its just to lower the throttle and try againg. Its like the vibration from one leg hitting the board starts an insane series of unbalance. !?
  2. That revup… why does this multicopter do that but on one else? Is it because of the much higher PIDs than other copters have?

Gonna film this next time he flies with it…

Other than that, it works splendid and is a workhorse that delivers. :smiley:


(Chris Olson) #27

That sounds like ground resonance like a helicopter can have due to the lead/lag regressive causing an imbalance in the rotor system and it starts reacting against the landing gear. It can do this to a helicopter:

I think it is possible for multicopters to experience it too. Say maybe if you have the PID’s tuned too tight so it rapidly throttles your motors side to side, setting up an imbalance similar to the lead/lag imbalance in helicopter rotors as they accelerate or decelerate to/from rated speed. And it starts to react off the landing gear and then feeds off itself if the landing gear’s natural frequency is the same as the frequency of the imbalance.

Since helicopters are required to do extensive ground running for some jobs, helicopter engineers go to great lengths in landing gear design and damping to prevent it. If it only happens on your wood board, but never happens on grass or soft surface, then your landing gear itself may be responsible for setting it off, and the PID loop starts to react to it and makes it worse. In which case you need to provide more damping on the landing gear, or don’t take off on a hard surface. Or de-tune your roll PID’s a bit.


(MickeM) #28

You are right, it works on grass, no hopping/shaking. Will make some soft dampers on the end of the legs and test again.

Number two, that insade revup is still an issue… but I guess we have to check on level soft surface to see what happens there too.

I remember now that there had to be special settings before doing an autotune on such a big mulicopter, which we did. And we have made several autotunes and they all work out fine. But then it was said here on ArduPilot to loosen up the tight values of the PIDs after autotune. I do not think we did that… :slight_smile:


(Shawn) #29

Stick some carpet on the wooden landing board, see if that provides enough damping.

Edit: we use a large carpet square as the launch and landing pad in grassy areas. It’s about 1m square.


(MickeM) #30

That is even better, will try that. Thanks!